Field of the Invention
The present invention relates to a synchronization pulse generator in a spread spectrum communication system, particularly to a synchronization pulse generator which generates a synchronization pulse for initial synchronization by using a surface acoustic wave convolver (hereinafter called SAW convolver) or the like.
Different from the narrow band communication which is conventionally widely used, the spread spectrum communication is the system for spreading the energy of information signals to a very wide frequency band. Therefore, this communication has various advantageous features which cannot be obtained in the conventional communication and can be applied to various wide fields such as space communication, ground communication (in particular, mobile transceiver), distance measurement, instrumentation, and the like.
The spread spectrum system includes the following systems.
(1) DS (Direct Sequence) system PA1 (2) FH (Frequency Hopping) system PA1 (3) TH (Time Hopping) system PA1 (4) Pulse coding FM system PA1 (5) Hybrid system
In general, at present, the DS and FH systems are used, the TH system and pulse coding FM system are applied to the limited fields, and the hybrid system is being theoretically studied. The principle of the DS system will now be described. On the transmission side, the information signal is subjected to an ordinary modulation (primary modulation). The primary-modulated signal is then modulated by the spread pseudo noise code (spread PN code) of a wide band and transmitted as a wide band signal having a very small power density. This operation is called a spread modulation. On the reception side, the correlation with the received or incoming signal is derived by use of the same demodulating PN code as that on the transmission side. After the correlation was obtained, only the signal to be received is converted into the original primary-modulated signal of the narrow band. The other signals and interference signal become the wide band noises having a small power density. Only a desired signal is extracted by a filter. The primary modulation can use the analog system such as FM and the digital system such as PSK. In general, the PSK system by the pseudo noise (PN) code is used as the spread modulation. The ratio of the band width between the primary-modulated signal and the signal after it was spread is called a process gain. As the process gain is large, the advantages of the spread spectrum system are obtained. In general, the process gain is set to 100 to 10000.
It is required that the demodulating PN code which is generated on the reception side has the same bit constitution and the same phase as those of the PN code in the incoming or received spread spectrum signal. Therefore, the initial synchronization (synchronization trapping) is performed to make the phase of the PN code on the reception side coincide with the phase of the PN code in the incoming signal. Next, in order to keep the phase-coincident PN code on the reception side, the synchronization holding process is performed by a delay-locked loop circuit (DLL).
In a conventional synchronization pulse generator using a SAW convolver as shown in FIG. 1, an input signal PN code and a reference PN code are supplied to input terminals of the SAW convolver to conduct convolution integration and detect a correlation, the reference PN code having a same PN pattern as that of the input signal within the range of a gate length of the SAW convolver and having an opposite time axis to that of the input signal PN code. The detected correlation is amplified by a correlation amplifier 2. An output from the correlation amplifier 2 is passed through a band-pass filter 3 to remove unnecessary frequency components and thereafter, it is subjected to envelope detection at an envelope detector 4. A detected envelope output is compared with a reference value at a comparator 5 to obtain a shaped waveform. The reference value is given as an output from a threshold setting rheostat 7. An output pulse from the comparator 5 is supplied to a synchronization holding circuit 9 as an initial synchronization pulse for use in controlling the circuit.
FIG. 2A shows a detected correlation output waveform from the SAW convolver 1, and FIG. 2B shows an output waveform from the envelope detector 4. The level of a detected correlation output varies with the level of an input signal by which a correlation was obtained. Thus, the level of an output noise becomes in proportion to that of the detected correlation output. When a level of the threshold setting rheostat 7 is set at Sa' as shown in FIG. 2B, an initial synchronization pulse has a waveform as shown in FIG. 2C, whereas when the level is set at Sb', an initial synchronization pulse has a waveform as shown in FIG. 2D. These initial synchronization pulses are used by the synchronization holding circuit 9 as a control pulse.
With the above-described conventional synchronization pulse generator, however, since the threshold value set by the threshold setting rheostat is fixed, the pulse width of an initial synchronization pulse varies as shown in FIGS. 2C and 2D as an input signal level of the SAW convolver changes. Further, the comparator 5 may generate a false initial synchronization pulse due to superposition of noises or the like so that the false pulse makes the synchronization holding circuit unstable.